Numerical hydrological and hydraulic simulation of the effects of a possible flood wave as part of a risk analysis and risk management at the Suggadinbach

Abstract

The Maesstobel bears high risks due to possible rockslides in the upper reach. Such a rock slide could result in a debris jam in the receiving stream 'Suggadinbach' caused by tumbling rock and debris flows of approximately 800,000 cubic metres. According to experts, dam failure and a consecutive flood wave would follow. The flood wave would then directly hit the town of St.Gallenkirch. In response to this scenario, a project has been designed which assesses the hazard potential and the effects of a possible flood wave on the residential area of St. Gallenkirch. Corresponding to the latest technical standards, laser scan data was used in the project to set up a digital terrain model. A detailed geological scan served as another basis. The hydrologic calculations were carried out with the aid of the HEC-HMS model. So as to know the extent to which dam failure would flood residential areas, various scenarios were assumed in hydraulic calculations and analyses. In order to increase accuracy and test plausibility, the simulation section was split into three sub sectors according to topographic and hydraulic aspects. In sector 1, debris flow in the Maesstobel was simulated (2D), involving its flow into the receiving river and the possible maximum height of retained debris and water. Sector 2 dealt with the scenario of dam failure, and it contains different variants of dam failure and the consecutive flood wave. In the third sector, there is the debris cone and the residential area of St.Gallenkirch. For this section, the worst case scenario was simulated (2D). Moreover, the threatened areas on the debris cone were defined. On the basis of these findings and results, a bundle of measures were taken as part of a risk management; also, a monitoring and early warning system was installed. Moreover, a new technical construction project, which contains three debrissorting dams, was carried out.